Shane Mosley an 18-month-old boy was brought to the emergency room by the baby sitter for treatment of a swollen and tender right knee that had developed suddenly within the previous three hours. The knee began to swell soon after Shane tripped on the family room carpet. Physical examination reveals an apparently healthy child who is crying and favouring his right leg. The knee is swollen and held in partial flexion. Shane has a few old, superficial bruises over shins, chest wall and his back. The physician in the ER concludes that there is fluid in the knee and because of the sudden onset and absence of fever, thinks this is most likely due to a joint bleed. The physician wonders about an underlying systemic bleeding disorder as the cause of Shane's joint bleed. A complete blood count, "hemostasis screen" and an x-ray of the knee are ordered.